Atma-Sphere M-60 Mk. 3.3 OTL Monoblock Amplifiers

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Tubed power amplifiers
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Atma-Sphere Music Systems M-60 Mk. 3.3

I connected the M-60 amps to my PS Audio BHK Signature Preamp which offers five balanced and five single ended connections. All connections used through the BHK Signature were balanced using all AudioQuest MacKenzie and Water XLR’s. 

Getting back to the tubes in the M-60, I asked Ralph why he selected the 6AS7 tube as the power tube of choice. Low cost and availability were one reason, but the other was he was looking for a tube with the lowest possible plate resistance. He considered the 7241 and the 6336 but they were expensive. The 6AS7 was inexpensive and available. Every few years he would do a price performance calculation on all the tubes that would fit his circuit profile. What stood out about the 6AS7 was that it was designed as a voltage regulator and those tubes have to be very linear. The more linear it is the more effective it is at voltage regulation. That type of tube needs to be a power triode. Unlike a pentode such as the KT88 which requires careful tube matching and regular biasing the 6AS7 has a mu of two. It is such a low value that its grid is relatively insensitive to minor variations. If you can set its bias point at the factory it is going to be in the right spot for the long term. The 7241 and 6336 can fill the same role but at much higher costs without any improvement in sound and performance. He bought as many of the 6AS7 tubes as he could from a few supplier’s he found and stocked up for the long haul. His house was packed with them for almost 20 years. Their reliability and low cost have allowed Atma-Sphere to put a full one-year warranty on the tubes of each M60! If you need more power than 60W in Class A Atma-Sphere makes amps that can scale up to as much as 500W into a three-ohm load using the same OTL circuit. However, I cannot imagine what size concert hall you would need, in order to need 500W of Class A power. 

Talk of warranties brings up a point about people’s beliefs around OTL amplifiers. The long-term thought has been that OTL amps blow up or are unreliable, largely because Julius Futterman’s designs from the 1950s did reliability issues. Seventy years later, the circuit designs of Atma-Sphere’s OTL have solved that challenge. Ralph says the most Sisyphean (his word, naturally I had to use it!) task he faced early on was simply convincing people his designs were stable and reliable. Forty years and no blow ups later, he has more than proven his point about how reliable a well-designed OTL can be.

In my time with the M-60 amps, they behaved admirably with no hiccups of any kind completely rendering the historical challenges of OTL amplifiers moot. 

Initially I connected the M60 Mk 3.3 amps to a pair of Zu Audio Omen Dirty Weekend speakers via a pair of Silversmith Audio Fidelium cables. The Omens are an easy load at 12 Ohms (Running off the 8 Ohm taps) and 97dB efficient. The M-60 pair delivered a sense of realism through the Omen’s that was wonderful. Listening to Pink Floyd’s Dark Side of the Moon on SACD [EMI] via my PS Audio DirectStream DSD DAC and SACD Transport The sense of scale was wonderful and the metallic tone of the clocks from the song Time rang out in all of their chime filled glory. I am unable to listen to just one track from this classic, so a full listen was required. Played through the M-60 mono amps, the delivery was as wonderful as I had heard it. The real stand-out track was ‘The Great Gig In The Sky’ featuring Clare Torrey’s incredible vocal pyrotechnics. The clarity and body were exceptional. Her emotion shone through making this a special listening event. It was this more visceral presentation that I enjoy from a high-end tube amp and the M-60 Mk 3.3 played so well. 

Zu Audio is generally the life of an audio show party so it was time to see how dynamic the M-60 amps could be with a pair of speakers that can really bring it for volume and clarity. Tower of Power cut a direct to disc Sheffield Labs album in 1981. The single What is Hip has a particular energy and realism brought by this live performance. Horns, guitars, drums drive this sonic powerhouse song. The M-60’s delivered more than enough power with the authority necessary to bring this tune to full life. The Zu Omen speakers brought the live performance to the room with floor shaking fun.

Swapping out the Zu Omen for a pair of DeVore Fidelity O/93, it was time for some finger picking with Tommy Emmanuel. The title track of Endless Road [Favored Nation] offers up expert playing with his Maton guitar. Air and space between notes are a hall mark of a great amp, while also offering up great tone. The M-60 amps delivered this in abundance via the O/93’s. The 10 Ohm 93dB efficient O/93 proved another excellent match for the M-60’s. The realism of fingers on strings was a delight and reminiscent of when I attended a Tommy Emmanuel concert with fourth row centre tickets. The M-60 amps made the listening an experience rather than a simple listening session. Most delightful.

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